Everybody mentions the big morale boost the US war entry had in April 1917. I do not doubt that, but I will also claim that Russia quitting in late 1917 must have been an even bigger blow for the Entente. USA's entry surely gave promisses of the future, but Russia quitting here and now removed a huge number of troops from the entente and heavily incresed the number available to the CP for deployment on the west front.
So no matter if USA had entered the war or not the question was if the Entente, on its own, could resist the comming German offensive with the forces "freed" from the East front. If the offensive could be stopped, it was obvious that Germany would not have the resources to do it again, but a US contribution to the counteroffensive would be a welcome aid to share the losses to be expected in the "final kill". If the comming German offensive could not be stopped, a US declaration of war from April 1917 would be of no use whatsoever as it would have extremely long prospects if any, before a US-British invasion of the European continent could be successfully launched, and UK anyway was safe from German invasion in any foreseeable future.
That offensive started in March 1918, at a time when there only was a few thousand US troops in France, and was stopped by April without USA giving any significant contribution. The US contribution only weighed in by Autumn 1918, when the Entente started to conterattack. Without the US forces I doubt the Entente by 1918 would have been able to have the German front dissolve, but the Fuller (armoured) offensive of 1919 might have done the trick. BTW the British units having suffered the most in the spring offensive (5th Army) were already by mid April back on near full strength in both men and materiel, showing that UK by this time did not have significant manpower or materiel problems. The French were not to the same degree in the firing line, but it appears like the French Army had overcome its 1917 crisis.
Some have proposed the effect of US finances on the Entente cause. They of course must have had an effect, as UK and France after the war owed money to various Americans, but I question to what degree that was dependent on a US declaration of war, and if the DoW had any effect I doubt it would be much before march 1918. A netral status rarely has meant not trading with the belligerent nations, I guess on the contrary, and that neutrality rather means that you can expect a higher profit on your prducts (incl. money) as there are no "special price for you my allied friend". In this context the Entente probably got some decords from the USA, but OTOH they would not have to supply the AEF with heavy weapons and planes as in OTL if USA stays neutral.
But USA's entry certainly had a very important impact - it made it possibe for the Entente to decisively beat the CP in late 1918 which again was the prerequisite for Versailles and after that WWII. The Fuller plan of 1919 might have achieved the same without US contribution, but I'm not sure if the Entente by 1919 would/could take the losses necessary to achieve the same degree of breakdown in the German army. The Germans would have been aware of that, and so an armistice on more balanced conditions (but still a Entente victory) would have been more possible - which again makes WWII less possible.
So all in all - the US entry in WWI created WWII (among other things) - which is quite ironic as Wilson had the best intentions. But the biggest crimes/errors are often made with the best intentions.
Regards
Steffen Redbeard